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Disconnector switching operations in large gas-insulated switchgear (GIS) systems may produce very fast transient overvoltages (VFTOs), causing high electric stress to other apparatuses in the substation, such as power transformers. This paper presents a VFTO measuring technique based on a transformer bushing sensor, which consists of the transformer bushing (C1) and a capacitor sensor (C2) installed at the test tap of transformer bushing. In order to solve the problem of the frequency limitation of the transformer bushing sensor for measuring VFTO, a convolution model on the square-wave response of the transformer bushing sensor is introduced to obtain the frequency response of the transformer bushing sensor, and a deconvolution model based on the incremental Wiener filter is applied to reconstruct the input transient voltage waveform at the tip of the bushing by using the output signals from the bushing sensor. A 110-kV transformer bushing sensor and a fast transient overvoltage source are built in our laboratory. The results from the laboratory indicate that a transient voltage can be measured correctly based on our technique with the 110-kV transformer bushing sensor. A 750-kV transformer bushing sensor is built in a 750-kV substation. The measuring results from the 750-kV substation by using our technique show that the peak value of the VFTO produced by the disconnector (AIS) switching off operations is about 775 kV, and the dominant oscillating frequencies of the VFTO are 3.5 MHz, 1.6 MHz, and 650 kHz.